1. Field of the Invention
The present invention relates to a method for controlling acceleration/deceleration of an induction motor and a control device, and more particularly, the present invention relates to a control process and a control device for automatically controlling the excitation frequency of an inverter in response to the load.
2. Description of the Related Art
As a method to control acceleration/deceleration of an induction motor, there can be mentioned an open loop control which controls acceleration/deceleration without requiring speed detection of the motor. As the open loop control, a primary frequency control is known. The primary frequency control is a method to change the synchronous speed and control the rotation speed by changing a frequency supplied to the induction motor. In this case, if the frequency is changed with the supplied voltage being constant, the internal magnetic flux changes in inverse proportion to the frequency. Therefore, in the low frequency region, the magnetic flux saturation is caused, on the other hand, in the high frequency region, such a phenomenon that the magnetic flux becomes short is caused. In order to avoid such a phenomenon, the supplied voltage is also changed so as to be approximately proportional to the frequency. Generally, the control of the frequency is carried out using an inverter. For example, the alternating voltage for the commercial power supply is converted to the direct voltage by a converter, which is converted to variable voltage and variable frequency set up with the voltage/frequency command by a general-use inverter.
In general, an inverter is based on a linear acceleration/deceleration. Therefore, in the case where the acceleration and deceleration are carried out by the inverter, if the excitation frequency is changed stepwise, the effective torque is not output, thereby the acceleration and deceleration are not carried out smoothly. For example, as shown in FIG. 11 showing the accelerated and decelerated time of the inverter, at the time of acceleration, the acceleration time is determined by a condition that (torque of the motor-load torque) becomes minimum, and other conditions such as the set rotation number of the motor, load inertia and the like. On the other hand, at the time of deceleration, the deceleration time is determined by the conditions such as (torque of the motor+load torque), the set rotation number of the motor, load inertia and the like. Therefore, considering the time required for the above-mentioned acceleration/deceleration, the excitation frequency is changed by a certain time constant. The time constant is generally preset corresponding to the load inertia and the like, and the excitation frequency is changed in response to the set time constant.
Therefore, in the conventional method to control the rotation speed of the induction motor, the excitation frequency is changed according to the set time constant. However, in this conventional method for controlling the rotation speed, when the load inertia and the like become large, even if the excitation frequency is changed with the preset time constant, the time constant cannot follow the excitation frequency. In such cases, there is a problem in that a proper time constant should be reset corresponding to the magnitude of the load inertia.